Grass mowing machines used to mow golf courses, parks, athletic fields, and other areas where quality and consistent cut height is important, may have cutting units attached to lift arms extending from a traction vehicle. Lift and lower mechanisms may be used to raise and lower the lift arms. Once a cutting unit is lowered to the ground surface, the weight of the cutting unit and lift arm may be used to maintain the cutting unit in ground contact during mowing operations. However, at relatively high travel speeds or rough terrain, the cutting unit has the tendency to bounce and effectively change the height at which the grass is being cut in relation to the ground. Additionally, thick grass or thatchy conditions can make a cutting units rise up, effectively changing the height of cut in relation to the ground.
To prevent or reduce variations in cut height due to bouncing of a cutting unit or heavy grass, some grass mowing machines include devices that apply a supplemental down force to each cutting unit. For example, U.S. Pat. No. 5,297,378 assigned to Deere & Company of Moline, Ill., relates to a suspension mechanism for reel mowers that uses hydraulic pressure on the lift cylinders to apply a down force to each cutting unit. U.S. Pat. No. 5,042,236 assigned to The Toro Company of Minneapolis, Minn., relates to a cutting reel suspension in which each lift arm is biased downwardly by a spring arrangement.
During mowing operations, these devices apply supplemental down pressure continuously to each cutting unit, even if the added force is unnecessary. One adverse effect of the continous application of down pressure is a higher draft load on each cutting unit, which requires higher mower power from the traction vehicle to push the cutting units across the grass. In conditions where supplemental down force is not needed, the extra power required to push the cutting units becomes a parasitic load. As a result, engine horsepower consumed for application of down force is unavailable for other important functions such as climbing steep hills.
A grass mowing machine is needed that allows selective application of downforce to a cutting unit, instead of applying downforce continously to the cutting unit. A grass mowing machine is needed that can reduce the engine horsepower required to push each cutting unit. A grass mowing machine is needed that can increase the engine horsepower available for hill climbing of the traction vehicle.